Research

The two best working theories we have so far for describing matter and its fundamental interactions are the Standard Model of particle physics and general relativity. The Standard Model represents matter and three of the four fundamental forces (electromagnetism, the weak force, and the strong force) as quantum mechanical, while general relativity (GR) explains gravity as a classical force arising from the curvature of space-time.

For years, it has been believed that coupling a quantum and a classical system would lead to incurable pathologies. Classical objects, in theory, can be known with infinitely high precision, whereas a quantum system must obey the Heisenberg uncertainty principle, limiting the amount of information that can be obtained about certain properties. At first glance, it seems that classical and quantum objects cannot coexist in the same Universe. If you naively try to put a classical and a quantum system in contact, you can always engineer the classical one to extract more information about the quantum object than is allowed by the rules of quantum mechanics. This has led to the widespread belief that GR must be a classical approximation to a fundamentally quantum theory of gravity that we have yet to discover. A significant portion of the community is dedicated to the quest for such a theory, with notable candidates including string theory, loop quantum gravity, and causal set theory.

However, this is not the full story. We have yet to observe any signature of the quantumness of gravity in a laboratory setting. If one is willing to adjust the definition of "classical," there is a way to couple GR to the Standard Model consistently. This requires introducing a certain amount of randomness in the evolution of the Universe, resulting in a loss of predictability. This class of "postquantum" classical-quantum (CQ) theories is the focus of my current research. Specifically, I am investigating potential indicators of such stochasticity at the cosmological level, i.e., in the large-scale structure of the cosmos. Through this approach, we hope to determine whether a classical theory of gravity can be ruled out by astrophysical observations.

One of the biggest achievements of string theory is uncovering what is commonly known as the holographic principle, or more technically, the AdS/CFT conjecture. The key idea is that specific classes of gravitational theories are encoded in (or "dual to") quantum field theories in one less spatial dimension, where gravity is absent. For instance, there is a belief that we can map questions about certain properties of gravity in 4-dimensional spacetimes with negative curvature (known as AdS spacetimes) to questions that can be answered in a specific quantum theory called a conformal field theory (CFT) existing in a 3-dimensional spacetime without gravity. Although the exact nature of this map is unknown, we have discovered various aspects of it. The overall validity of this conjecture remains unproven, but we are aware of specific instances where this seemingly implausible duality holds true.

In another line of my research, I utilize tools from the AdS/CFT correspondence to investigate what happens to black holes in the presence of a quantum field. Specifically, I examine how black holes, which are forbidden in certain lower-dimensional empty spacetimes, can actually form when the effects of quantum fields are taken into account.